The present invention relates to an arrangement for discharging medium consistency pulp in connection with different pulp treatment devices or apparatuses. Washers used for washing pulp are disclosed below as an example.
Several types of washing apparatuses are known from the prior art. Known arrangements include diffusers, drum washers/disc washers and Fourdrinier washers, which clearly differ from each other. Pulp is fed into diffuser washers at a consistency of 10%. The feed consistency for drum washers and Fourdrinier washers is normally between 1 and 3%. Drum washers presently used are, for example, suction washers, wash presses and pressure washers.
When a conventional suction washer is used, the starting consistency of the fiber suspension in the vat is about 0.5-2%, and the consistency of the pulp layer thickened on the drum is about 10-12%.
A washer press receives the pulp at a consistency of 3 to 4%. The consistency of the washed pulp may rise even to 30%, when a press roll is used.
An example of a pressure washer is mentioned an apparatus in accordance with Finnish patent application Ser. No. 854287, published Apr. 31, 1987, now Finnish U.S. Pat. No. 71961 (corresponding U.S. application Ser. No. 921,786, filed Oct. 21, 1986), which discloses a so called drum type washer. The pulp is fed into the apparatus normally at a consistency of 3 to 6%, but in case the feed means described in the parent patent of this case or in the copending application Ser. No. 7/558,142, are used, the pulp may even be fed at a consistency of 8 to 20%. After the last washing phase the "pulp planks" are removed from the drum reaching a consistency of 15 to 17%. The washing water flowing from the compartments back on said planks, however, dilutes the consistency to 10 to 12%.
Also, disc type filters may be used for thickening or washing fiber suspensions. Their operation with respect to feed and discharge consistencies equals of pressure washers, except that a medium consistency feed for a disc filter has not yet been developed as far as a commercial product.
All these apparatuses are characterized in that the treated pulp is most often discharged from the treatment apparatus by means of allowing the pulp to drop freely along a chute into a drop leg, most often used when the treatment apparatus is substantially short, or into a trough having a screw conveyor on the bottom. The purpose of the screw conveyor is to move the pulp, often in the form of "planks", to the other end of the apparatus (the length of which may exceed 5 m) and let the pulp drop into a drop leg, the height of which may be several meters. The medium or high consistency pulp is then pumped from the bottom of the drop leg forward preferably by a fluidizing, so called MC pump (manufactured and marketed by AHLSTROM PUMPS Inc., Peace Dale, RI). It is to be noted, however, that there are some special arrangements for applying an ordinary centrifugal pump, i.e. a pump having no fluidizer, for pumping medium or high consistency pulps from a drop leg.
Thus, a characterizing feature to all prior art pulp treatment devices is the fact that the pulp is discharged into a drop leg, which is several meters high, most usually between 5 to 10 m, to ensure sufficient pressure in the suction opening of the pump for a successful pumping. Because the pulp treatment apparatuses are most usually all at the same level in a pulp mill, it has always been necessary to pump the pulp first back up from the bottom before it has been possible to feed it to the next treatment apparatus.